Pulsation-induced errors in orifice meters and other differential pressure flow measuring devices have plagued measurement personnel for decades. The quantitative determination of pulsation-induced errors on a sound, and generally acceptable, basis has been extremely difficult, if not altogether infeasible. Thus, pulsation-induced errors have led to many problems. These problems are critical when, for example, the custody transfer of materials is involved. Custody transfer problems are critical when two meters in series do not agree and the quantities or unit costs of the material being transferred represents a large amount of money. In the natural gas industry, as well as other industries which require the control of fluids, orifice meters are the flow measurement standard. Hence, the achievement of acceptable measurement accuracy is a basic requirement since not only is the value of the transferred fluid a consideration but, also, such meters represent both a large investment and establish the basis and procedures upon which custody transfer and system balances depend.
Numerous theories and explanations have been advanced upon which pulsation-induced errors can be argued and remedies postulated. For example, valid explanations have been presented to explain the "square root error," lead line distributed resonances, lumped-element resonances, signal rectification, phase differences, energy redistribution, and various methods of correcting chart integration and/or electronic flow computation. Many of these are valid and useful in dealing with specific components of the measurement system or process. No generalized explanation of pulsation-induced error, using any one or more of these accepted explanations, has resulted either in the quantitative explanation of the totality of phenomena for the entire measurement system or in a means for the quantitative determination of total system pulsation-induced error.
To further complicate matters, the frequency spectrum and amplitude of pulsations at any point in the measurement system are functions of compressor speed, fluid temperature and other operating and ambient variables. Consequently, any measurements or error diagnoses made are condition-specific. Projections of pulsation conditions and induced errors for the entire range of anticipated operating and ambient conditions, therefore, would require a large number of expensive measurements which are difficult to interpret. It is pertinent to note that the establishment of coefficients for orifice meters, for steady flow conditions only, has been under way for decades and is still being investigated for situations where greater flow measurement accuracy and validity are being sought. In particular, such validity and accuracy are being sought for wider ranges of gas properties and operating conditions.
The most widely accepted means for reducing pulsation-induced errors in orifice meters has been that of isolating the meter from pulsations by the insertion of acoustic filters (pulsation dampeners) or by remote location of the meter station from compressors and other sources of pulsation. However, isolation is a predictable and effective solution only insofar as the primary element of the meter is concerned. Pulsations do not "die out" or damped quickly in a transmission line for fluids of low viscosity and, therefore, pulsations travel long distances. Thus, isolating the meter from pulsations by placing the meter in a remote location is not only expensive and inconvenient but, also, may prove ineffective in some instances. Similarly, for specific situations, isolating the meter from pulsations by the insertion of acoustic filters can be effective. However, in high pressure large diameter transmission lines, the installation of acoustic filters is expensive either in terms of the capital investment or in terms of operating cost due to the introduction of a pressure drop. Further, "isolation" is not necessarily possible since pulsations are generated by the flow itself when passing junctions or constrictions such as valves. In most cases, some level of pulsation is present to excite resonances in both the primary and secondary elements of the meter system which can then result in significant meter error. A gas-filled pipe is a finely tuned resonator with little internal damping. Thus, little energy is required to excite a gas-filled pipe into high amplitude resonance. The amplitude of resonance is obviously greater when the frequencies of the exciting pulsation and the resonator closely coincide. Further, the exciting pulsation usually includes many harmonics of the basic frequency, such that coincidence is often realized.
In general, a differential pressure flow measuring device consists of (1) a primary element, such as an orifice plate or pitot tube, that creates a differential pressure that is proportional to flow rate in a known and repeatable way, and (2) a secondary system which includes a conduit for the transmission of the differential pressure so generated from the primary element to a transducer, the transducer and a recorder or flow computer. When the differential pressure includes a pulsating (dynamic) component, each of these elements of components can produce an error or change an error in the measurement of flow rate and total flow.
Any effective discussion of pulsation-induced errors requires reference to a specific meter type. Because of the long time prevalence of orifice meters in the natural gas production and transmission industries and many process industries, the orifice is used as the basis for the following discussions. Analogous descriptions and discussions would apply to other differential pressure sensing devices, including flow meters. For orifice meters, it should be noted that the orifice plate may be held between flanges, or in a fitting, or the like so long as dimensions, alignment, concentricity, tap hole sizes and locations, and other prerequisites are in accordance with specified standards (e.g., AGA3--1985). In the present discussions, the terms flange and fitting shall be used interchangeably depending on the specific discussions. Similarly, the terms differential pressure sensing device, orifice meter or flow meter shall be used interchangeably depending on the specific discussion.
The primary element in an orifice meter includes circular, concentric, smooth tubing of prescribed length upstream and downstream of an orifice plate and the associated flanges or fittings required for meter installation or for the orifice plate replacement or inspection. Generally, the primary element of an orifice meter is very insensitive to pulsation. For example, a severe pulsation of forty percent (40%) pressure amplitude is required to produce a "square root error" of one percent (1%), and a pressure amplitude of thirty percent (30%) is required to produce a "square root error" of one-half percent (0.5%). The "square root error" in the primary element is a predictable and calculable value that is based on the dependence of differential pressure on the square of the velocity in the primary element. The "square root error" can be computed by: ##EQU1## where: SRE.sub.f is the fractional square root error in flow,
.DELTA.P(t) is the time varying differential pressure.
The square root error can be measured accurately in the field with suitable instrumentation. However, the square root error measurement is quantitatively meaningful for the primary element only (i.e., when made directly at the tap holes in the orifice flange or fitting). While a "square root error" can be measured at points in the secondary system, it is not quantitatively meaningful since there is no net flow in the secondary system and, thus, the appropriate relationship does not exist. Nonetheless, such measurement in the secondary system are useful in determining both the presence of significant square root error in the total measurement system and in diagnosing the cause for any change in the square root error between the primary element and the transducer.
In addition to the square root error, the primary element is theoretically subject to an "inertial error." An inertial error measurement evinces changes in the orifice coefficient used to calculate flow from differential pressure that results from pulsation. However, experiment shows that such errors are insignificant when the square root error in the primary element is one percent (1%) or less, and, the pulsation frequencies in the primary element are less than 100 Hz. These conditions are seldom exceeded in the field and, in any event, corrective action would be dictated by the magnitude of the square root error regardless of any shift in orifice coefficient. Hence, the inertial error appears to be of no practical consequence.
In the secondary system, the means for transmitting the differential pressure generated in the primary element to a transducer is normally an acoustic transmission line. Generally, an acoustic transmission line comprises lead lines, valves, manifold and the like. Hence, any acoustic mismatches or resonances in the transmission line can result in the distortion or amplification of the dynamic component of the differential pressure, and can result in a shift in the average value of the static differential pressure from that corresponding to a steady flow without pulsation. One objective of the present invention is the design of the transmission line so that the differential pressure generated by flow in the primary element is transmitted from that element to a transducer without distortion, amplification or shift in average value. It is the transmission line portion of the secondary system with which the present invention is concerned. However, the interaction of the transmission line portion of the secondary system with the primary element and with the other parts of the secondary system must be an integral part of the design of any metering system.
The transducer can also contribute to a pulsation-induced error in flow measurement using differential pressure devices. For example, the frequency response of the transducer can be, and frequently is, very low. Thus, most of the dynamic component of differential pressure cannot be sensed by the transducer. This can be a desired effect provided the transmission line and transducer characteristics are designed for a common objective (for example, if the transmission line does not change the average differential pressure) and this is, also, compatible with the recorder or computer characteristics. It this common objective is not realized, however, this effect is generally detrimental to maximum measurement accuracy. Also, the transducer may participate in a mechanical/acoustical resonance with the transmission line, particularly if its mechanical resonant frequency coincides with that of the pulsation present in the primary system or with resonant frequencies in any part of the acoustic transmission line. The latter can be a major contributor to measurement error.
A mechanical recorder can enter into a common mode resonance with the acoustic transmission line as in the case of the transducer. However, the mechanical recorder entering into a common mode resonance with the acoustic transmission line is rare unless the transducer is a direct drive for the recorder pen. An electronic flow computer, however, is free of any acoustical/mechanical type of error generation. Nonetheless, most electronic flow computers sample the differential pressure on a periodic basis (e.g., once per second) and calculate flow from the sampled value of differential pressure. Hence, the presence of either a dynamic component in the transducer output (corresponding to the pulsation in the primary element, or to resonance, distortion or amplification in the secondary system) or an average value shift caused by the transmission line can result in errors in the computation of flow rate and total flow.
The secondary system of the orifice meter is the predominant source of significant pulsation-induced errors. The secondary system can increase the pulsation-induced error extant at the output from the primary element by one or two orders of magnitude. Within the secondary system, the acoustic transmission line is the major factor to be considered. However, as stated, the interaction of the acoustic transmission line with the transducer and the recorder/computer should not be ignored when large pulsations exist within the secondary system.
No net flow exists in the acoustic transmission system, i.e., beyond the tap holes in the orifice flange or fitting. Pulsations are acoustic waves which ingress and egress the transmission system at the velocity of sound. The pressure and velocity of these acoustic waves are not in phase in any resonant section in the transmission system. Thus, acoustical impedance is a dominant factor in controlling the transmission of pulsations from the orifice fitting to the transducer. Generally, acoustical impedance and changes in acoustical impedance in the lead lines, valves, manifold and other components comprising the transmission system can cause pulsation-induced errors. More particularly, the acoustical properties of the system can amplify and distort even minor pulsation at the system input such that the pulsation is significant upon arrival of the signal at the transducer. Also, changes in acoustical impedance produce shifts in the average differential pressure prior to arrival at the transducer.
Acoustical impedance changes, or "mismatches," occur in both the upstream and downstream lead lines in the secondary system of an orifice meter. At any such mismatch, there is a partial reflection of energy in the dynamic component of the differential pressure signal. The positive or negative half of a sinusoidal pressure wave is preferentially transmitted versus the other half depending on whether there is an increase or decrease in impedance. The result is partial rectification of the wave with a commensurate shift in the average value of differential pressure. Also, even without impedance changes, a redistribution of energy between kinetic and potential energy is undertaken as the wave progresses through a transmission system. The redistribution of energy also results in a shift in the average differential pressure. Since most in place systems use a low frequency transducer and a flow computation based on the square root of the average differential pressure, either effect introduces an error in measurement which is an entirely valid error. These effects also invalidate a measurement of the square root error per se, since such measurements are based on the difference between the average of the square root of dynamic differential pressures and the square root of the average differential pressure. However, locations exist at which rectification cannot be avoided. For example, the change in cross section that occurs in the lead line entrance into a meter tube is unavoidable. It has been found that rounding off the junction at the inside diameter of the meter tube greatly reduces the rectification. However, only a slight rounding off of the junction at the inside diameter of the meter tube is allowed by regulation (A.G.A. 3--1985). Also, an unavoidable change in cross section occurs at the input to the transducer. Even if the transducer input is designed to match the diameter of the lead line, the acoustical path must terminate at the differential pressure sensing surface (diaphragm, solid state sensor or the like) of the transducer.
Any gas-filled constant diameter section of pipe is an excellent resonator. Such gas-filled pipe exerts only a small amount of damping on a standing wave. Thus, the pipe emulates an organ pipe and will resonate at a high amplitude at frequencies (harmonics) dependent upon the pipe length, types of end terminations, and the speed of sound. When these resonant frequencies coincide with the driving frequency or a multiple thereof, very high amplitudes will result. It is characteristic of most practical and/or available systems that compressor-generated frequencies (at some shaft speed) often coincide with the resonant frequencies of some length of compressor or meter station piping and with lengths within the primary or secondary elements of an orifice meter. In combination with flow-generated pulsations, this means that pulsation is present in most practical systems and that resonance at high amplitude is routine and prevalent.
When high amplitude resonance occurs in the secondary system, the resulting high dynamic pressures and particle velocities exaggerate such effects as rectification and energy redistribution. Hence, shifts in the average differential pressure are larger and create more serious errors. It should be noted that, in most installations, the upstream and downstream lead lines, valving and the like are nearly symmetrical so that these effects should be comparable in the upstream and downstream acoustic transmission lines. Such effects might, then, tend to balance. However, the driving frequencies and amplitudes at the upstream and downstream tap holes are not the same such that both the frequency spectrum and the amplitudes of resonances in the two would normally be quite different and such balancing would not occur.
From the preceding discussion, it is apparent that long lead lines in the secondary systems of orifice meters and other types of differential pressure meters often resonate at frequencies that coincide with compressor and flow-generated pulsations and, thus, give rise to severe pulsation-induced errors. Therefore, to avoid coincidence with driving frequencies, the lead lines must be as short as possible. However, measurement operations required a multiplicity of valves in the lead lines for isolation, zero-adjustment, calibration and similar related operations. The valves are discontinuities which create impedance mismatches that shift the average differential pressure and distort and amplify the dynamic component of differential pressure. There is a need, then, for a unique multi-valve manifold coupled with a unique flange/fitting insert for use in conjunction with an orifice meter as well as other differential pressure flow measuring devices for minimizing the pulsation-induced errors.
It is, therefore, a feature of the present invention to provide a unique apparatus and method for minimizing pulsation-induced errors for implementation with orifice meters and other differential pressure flow measuring devices. It is a more particular feature of the present invention to provide an apparatus and method for minimizing pulsation-induced errors to prevent any significant increase in the square root error associated with the secondary system of an orifice meter or similar differential pressure flow measuring device.
Another feature of the present invention is to provide an apparatus and method for minimizing pulsation-induced errors for use in conjunction with an orifice meter or other differential pressure flow measuring device and a transducer such that the secondary system maintains custody transfer accuracy under all practical ranges of operating and ambient conditions.
Yet another feature of the present invention is to provide an apparatus and method for minimizing pulsation-induced errors for use in conjunction with an installation including an orifice meter or other differential pressure flow measuring device in conjunction with a transducer such that using a low frequency transducer and a flow computation based on the square root of the average static differential pressure provides custody transfer accuracy under reasonable combinations of operating and ambient condition.
Yet still another feature of the present invention is to provide an apparatus and method for minimizing pulsation-induced errors for use in conjunction with an installation including an orifice meter or other differential pressure flow measuring device and a transducer to prevent the installation from going into resonance by eliminating any resonant lengths that coincide with or approximate frequencies present in the primary system.
A further feature of the present invention is to provide an apparatus and method for minimizing pulsation-induced errors for use at an installation including an orifice meter or another differential pressure flow measuring device and a transducer for reducing the effects of rectification and energy redistribution of the acoustical signal within the secondary system to minimize shifts in the average static differential pressure.
It is also a feature of the present invention to provide a unique insert apparatus which constitutes an extension of a unique multi-valve manifold such that, in conjunction or alone, a constant cross-section is maintained within and at all times egressing from an orifice flange or fitting and thereby enhancing the capabilities of the installation by (1) eliminating rectification in the secondary system to avoid shifts in average static differential pressure, (2) maintaining custody transfer accuracy under practical ranges of ambient and operating conditions, (3) avoiding the coincidence of pulsation frequencies in the primary system and resonant frequencies in the acoustic transmission component of the secondary system, (4) preventing high amplitude resonance in the acoustic transmission component of the secondary system and (5) minimizing the effects of energy redistribution on the average static differential pressure in the secondary system.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims.